1. Field of the Invention
The present invention relates to a catalyst, suitable for purifying exhaust gas, that is used in removing nitrogen oxides in exhaust gases, particularly those from internal combustion engines such as used by automobiles, and a method of removing NO.sub.x by using said catalyst.
2. Description of the Prior Art
In view of the prevention of global warming, in recent years internal combustion engines of a lean burn system that can suppress the emission of carbon dioxide, for example, lean burn gasoline engines and diesel engines, have attracted notice. The exhaust gas from this type of internal combustion engine contains reducing components, such as carbon monoxide (CO), hydrocarbons (HC), and hydrogen (H.sub.2), as well as oxidizing components, such as nitrogen oxides (NO.sub.x) and oxygen (O.sub.2), and is characterized by an excess of oxygen and the presence of steam as a product of the combustion.
Conventionally, there are many reports that, as a catalyst of purifying this type of exhaust gas, a catalyst consisting of, for example, alumina or a transition metal on alumina is effective for the NO.sub.x reduction reaction that uses hydrocarbons as a reducing agent. Examples of Japanese Pre-examination Patent Publication (KOKAI) No. 4-90826 report use of powdered alumina for FCC as a NO.sub.x reduction catalyst. Also Japanese Pre-examination Patent Publication (KOKAI) No. 4-284848 reports examples wherein use is made of, as a NO.sub.x reduction catalyst, alumina or alumina/silica containing 0.1 to 4% by weight of Cu, Fe, Cr, Zr, Ni, or V. Further, for example, Japanese Pre-examination Patent Publication (KOKAI) Nos. 4-267946, 5-68855, and 5-103949 report that where a catalyst carrying Pt on alumina is used, the NO.sub.x reduction reaction proceeds in a low temperature range of about 200.degree. to 300.degree. C. However, where these catalysts having supported noble metals are used, the combustion reaction of hydrocarbons that are a reducing agent is facilitated and therefore the selectivity of the NO.sub.x reduction reaction is poor, which is a defect.
Some of the inventors of the present application found previously that where a catalyst containing silver is used in an atmosphere having excess oxygen with hydrocarbon used as a reducing agent, the NO.sub.x reduction reaction proceeds selectively, which was disclosed in Japanese Pre-examination Patent Publication (KOKAI) No. 4-281844. After this disclosure, similar NO.sub.x reduction techniques that used a catalyst containing silver were disclosed in Japanese Pre-examination Patent Publication (KOKAI) Nos. 4-354536, 5-92124, and 5-92125. Further, Applied Catalysis B: Environmental 2 (1993) 199-205 reports that an alumina catalyst carrying silver gives better NO.sub.x reduction performance in the copresence of steam than an alumina catalyst carrying Co, Cu, V, or Cr.
However, these conventional silver supported on alumina catalysts are still unsatisfactory as a catalyst for the NO.sub.x reduction reaction by hydrocarbons in the presence of steam.
Further, conventionally it is known that the catalyst which uses alumina as a carrier has high dependency on space velocity. That is, it is commonly known in the art and there have been many reports on that catalysts which use alumina as a carrier exhibit satisfactory NO.sub.x reduction performance at a low space velocity of about 1,000 to 10,000/hr, however, the NO.sub.x reduction performance drops greatly at a high space velocity of 10,000/hr or more, particularly in the presence of moisture. Because of this reason, the method of treating exhaust gases that is disclosed in Japanese Pre-examination Patent Publication (KOKAI) No. 5-92124, for example, restricts the contact time of the exhaust gases with the catalyst to 0.03 g.sec/cm.sup.3 or more, and preferably to 0.1 g.sec/cm.sup.3 or more. Japanese Pre-examination Patent Publication (KOKAI) No. 6-71175 discloses a NO.sub.x removal catalyst carrying silver or a silver oxide on a porous inorganic oxide having a specific surface area of 30 m.sup.2 /g or more and gives alumina as an example of the inorganic oxide. Although the Japanese Pre-examination Patent Publication (KOKAI) No. 6-71175 describes that the practical NO.sub.x removal performance is such that the contact time with exhaust gases is 0.3 sec.g/cm.sup.3 or less, in the Examples the shortest contact time with exhaust gases is practically only 0.05 sec.g/cm.sup.3.
In the treatment of exhaust gases of lean-burn engines used in vehicles, such as automobiles, that are typical internal combustion engines which are operated at a fuel lean air/fuel ratio, another practically essential factor involves the required space and the weight of the structure comprising a catalyst layer or a support substrate coated with a catalyst (which structure is referred hereinafter to as a catalyst-containing layer). This is because, taking the displacement of an engine and the work done by it together into consideration, it is not practical to mount a catalyst-containing layer having a volume that is several times or more the displacement of the engine. Therefore, generally, it is preferable that the volume of a catalyst-containing layer is equal to or smaller than the displacement of an engine. This means that a practical catalyst-containing layer is required to exhibit satisfactory NO.sub.x reduction performance even under conditions wherein the contact time between the catalyst and exhaust gas is very short, for example, less than 0.03 g. sec/cm.sup.3, and preferably at most 0.02 g. sec/cm.sup.3 (since a practical gas space velocity is over 7,000/hr, and preferably 10,000/hr or over.). However, the NO.sub.x removal performance of conventional silver supported on alumina catalysts with regard to exhaust gases containing steam is unsatisfactory at such a short contact time.